In general, a mask pattern forming technique has an important effect on the accuracy of patterns formed on a semiconductor substrate. In particular, if an optical proximity effect of the mask pattern is not suitably treated, unwanted line width distortion may occur between lines during a lithographic exposing process and a linearity of the line width may be deteriorated. Therefore, characteristics of the semiconductor device may be degraded.
On the other hand, by using semiconductor photolithography, masks can be designed with a high accuracy, so that an amount of light passing through the masks can be suitably adjusted. In addition, an optical proximity correction technique and a phase shifting mask technique have been proposed. In addition, various approaches for minimizing light distortion effects due to mask patterns have been proposed.
Recently, a chemical amplifying resist has been developed. The chemical amplifying resist is sensitive to 248 or 194 nm extreme ultra-violet (EUV) light. By using the chemical amplifying resist, the resolution of the mask pattern can be further improved. In particular, by using dummy patterns separated from the main patterns, the optical proximity effect can be controlled to improve the resolution.
In addition, in order to form an ion implant layer, the associated resist must have an ion implantation blocking fiction as well as a photosensitive function. Therefore, a highly viscous photosensitive material is used for the resist. In particular, for a well ion implantation mask, the breakdown voltage must be stably maintained by adjusting the impurity ions during the processes for forming N-wells and P-wells.
A method of forming an inter-well region between the N-well and the P-well and of selectively overlapping the two wells has been widely used. In order to implement the method, the sizes of the N-well and the P-well must be adjusted. Namely, all the N-wells and the P-wells are adjusted to have the same size, and individual inter-well regions between the wells must be overlapped. Here, it is very important to implant the ions symmetrically.
FIG. 1A is a plan view of a conventional semiconductor device. FIG. 1B is a cross sectional view of the semiconductor device of FIG. 1A. Referring to FIG. 1A, a P-well region 20 and an N-well region 30 are partially overlapped in an area (D2). Next, N+ and P+ source/drain regions 60, 50 are formed. Next, contact holes 70 are formed on the source/drain regions 50, 60.
FIG. 1B is a cross sectional view taken along line A-B of FIG. 1A. Insulating regions 4 are formed in the semiconductor substrate 1 by using an STI (shallow trench isolation) process. An N-well 3 and a P-well 2 are formed between the insulating regions 4. The N-well 3 and P-well 2 of FIG. 1B overlap to define an overlapping region D3. In the illustrated example, reference numerals 5, 6, 8, and 9 indicate source/drain regions, an insulating layer, and a contact, respectively.
FIGS. 2A and 2B are cross sectional views showing conventional N-well and P-well masks, respectively. Referring to FIG. 2A, the N-well mask 33 includes a light-blocking layer 12 and a light-transmitting layer 11. In the example of FIG. 2A, a reference line 99 indicates an end portion used to adjust the size of the N-well. In addition, size-adjusting regions are indicated by W. FIG. 2B is similar to FIG. 2A except that FIG. 2B shows the P-well mask 34. Therefore, a detailed description of the P-well mask would be redundant to the above description of the N-well mask and is, thus, omitted. However, like structures in FIGS. 2A and 2B are referenced with like reference numerals to easily enable the reader to apply the discussion of FIG. 2A to FIG. 2B.
FIG. 3 is a picture illustrating a problem of a conventional mask. When the conventional mask is applied, the photosensitive material has a large thickness of about 1.2 μm. Therefore, in the subsequent processes of forming an N-well layer 300 and a P-well layer 200, the slope of the resist is too slow to uniformly expose the inter-well regions 90a, 90b corresponding to the regions D1 and D2 of FIG. 1A.